A permanent-magnet rotating machine is structurally categorized into a radial gap type and an axial gap type. In the radial gap type, a plurality of permanent magnets are arranged in a circumferential direction of a rotor, magnetic poles of the permanent magnets are aligned in a radial direction, and a stator is arranged in a manner to face the permanent magnets. Generally in the radial gap type, the individual stator coils are wound around an iron core having a plurality of teeth, so that magnetic fluxes from the rotor poles can efficiently link with the coils, and a large torque for a motor and a large voltage for a generator can be generated. In contrast, since an iron core is used, there is caused a loss in torque based on a cogging torque or a hysteresis loss of the iron core, and, thereby, a starting torque is increased. If such a structure is used for a wind power generator and the starting torque is too large, the generator can not be rotated by a weak wind and can not generate electricity.
If the iron core is removed, such a problem is not caused. This, however, causes the magnetic efficiency to deteriorate, and it is impossible to obtain a large output in the radial gap type. In view of this, an axial gap type as shown in FIG. 8 is proposed.
In FIG. 8, a plurality of layers of disk-shaped magnetic bodies (rotor yokes) 25 comprising a plurality of permanent magnets 26a on the surface thereof are secured together through spacers to be fitted to a rotating shaft 22 and integrally formed therewith, resulting in a formation of a rotor 27. In air gaps formed between individual rotor yokes, a stator 1 comprising coils 3 is disposed and fixed to a housing 21. The rotating shaft 22 is rotatably supported by the housing 21 through bearings 28. According to this structure, it is possible to increase the output by enlarging the magnetic pole surfaces of the permanent magnets 26a without using iron cores for the coils 3. Since the axial gap type rotating machine has a structure using no iron cores for the coils (called “coreless”), it is possible to obtain a high-output rotating machine as it has no starting torque (for example, refer to such publications as Japanese Patent Provisional Publication (JP-A) No. 2002-320364 and JP-A No. 2003-348805). According to the coreless rotating machine, it has a smaller internal loss and offers a high output and highly efficient performance even at a high rotation speed, because the inductance of the winding is small, which makes the impedance thereof low. For example, when the coreless rotating machine is used as a coreless power generator, since the internal loss is a result of the product of the impedance and the current, it is possible to supply therefrom a larger current as the impedance thereof is smaller. The coreless rotating machine has an advantage for such an application to which a larger current is supplied. However, it is to be noted that the magnitude of current that is allowed to flow is decided by a cross-sectional area of the winding wire as the heat generation in the coil should be restricted and is supposed to be restricted at 5 to 15 A per 1 mm2 cross-sectional area of the winding wire.    Patent Document 1: 2002-320364A    Patent Document 2: 2003-348805A